No. 3.] AMPHIOXUS AND THE MOSAIC THEORY. 585 



specimens still show a more or less distinctly marked bilater- 

 ality when viewed from the lower pole. On the upper half of 

 the embryo the bilaterality soon disappears; the planes of 

 division in the micromeres conform to no general law and ap- 

 pear to be determined by individual mechanical conditions of 

 environment. 



T\i& fifth cleavage (16-32) is shown in Figs. 33 to 40, and in 

 Figs. 15 and 16. The eight new micromeres of the second 

 group (a3-d,3 a--^-d~-^) are formed by horizontal divisions. 

 Those already present divide in most cases in planes approxi- 

 mately parallel to the equator, as Hatschek describes, but there 

 are many individual exceptions. The bilateral symmetry is often 

 very striking still. 



The sixth cleavage (32-64, Figs. 41 to 44, and Figs. 16, 

 17) resembles the last in the origin of the eight new micromeres 

 of the third group {aA-d,A a^-^-d^-^). Most of the existing 

 micromeres divide in meridional planes (Fig. 42), but there are 

 many exceptions. It is noteworthy that one or more of the 

 primary macromeres may divide equally at this cleavage {e.g. 

 D-d,^ in Fig. 17), but this may be followed by an unequal 

 division in the following stage (Fig. 18). 



The seventh cleavage (64-128, Figs. 47, 45 and Fig. 18) 

 gives rise to eight micromeres of the fourth group {aS-d,S a-'i- 

 d'^-i), while the remaining micromeres divide equally in the 

 most diverse planes. 



The eighth cleavage (128-256) still shows in many cases 

 unequal divisions of the macromeres, a distinct group of which 

 surround the lower pole in the 256-celled stage (Fig. 46). 

 There are, however, many variations, and I have not been able 

 to trace the history in detail beyond this point. 



In the succeeding stage (approximately 5 1 2-celled, Figs. 48, 

 49) the blastula in some cases shows the first flattening of the 

 lower hemisphere preparatory to the invagination. In some 

 cases the cells still show traces of the bilateral arrangement 

 (Fig. 48) and occasionally the cleavage-pore is present. 



I shall not give in this paper a detailed account of the gas- 

 trulation, but will briefly call attention to a few leading points, 

 bearing on the mode of invagination. The cleavage-pore, which 



